Human tumor cells currently carried in culture, as well as cells obtained from patients at surgery, will be examined for their capacity to stimulate allogeneic lymphocytes. Tumor cells will be classified as stimulatory, (S plus), or non-stimulatroy, (S minus), depending upon whether they induce a response against themselves in allogeneic mixed leukocyte cultures. We will attempt to induce stimulation by S minus cells through the addition to cultures of a factor isolated from the supernatant of lectin stimulated lymphocytes-monocyte populations (co-stimulator). This has already been done for the S minus colon tumor, HT29. Next, the functions of subclasses within the responding cell population will be examined. Counterflow centrifugation will be used to isolate pure populations of small lymphocytes, large monocytes and an intermediate population consisting of a mixture of large lymphocytes and small monocytes. The effects on the cytotoxic response of varying the ratio of the relative numbers of small lymphocytes and large monocytes will be studied, along with the perturbations created by the intermediate fraction. We have already established that a stronger cytotoxic reaction is mounted against the S plus Lymphoma, RPMI 8866, when the intermediate fraction is removed from the responding cell population, perhaps indicating the presence of supressor cells. If S minus tumors removed at surgery can be converted to S plus by costimulator, the ability of the leukocytes of the tumor bearers will be examined for their ability to produce that factor. We have recently demonstrated that a costimulator preparation made from the cells of a normal donor appeared to contain a supressor substance which prevails over the stimulatory factor at high but not at low concentrations. Changes in the properties of costimulator will therefore be examined, for both normal donors and cancer patients, when the cell populations from which it is prepared are selectively depleted of their various components.